The present invention relates to thin film deposition, and, in particular, relates to an apparatus and method of making stand-alone thin films.
Most of the single crystal semiconductor wafers produced to date, including silicon and gallium arsenide, have been manufactured by crystal growth techniques that rely on melting of the material. In the Czochralski technique, for example, the purified semiconductor material is first melted in a suitable vessel. Next, a seed crystal is dipped down into the melt and slowly withdrawn. If everything is done right, a long cylinder, called a boule, of the single crystal material is obtained. The boule is sliced up into many thin wafers which are then polished to get the wafers into a usable form for device manufacturing.
An alternative method of growing thin crystalline wafers of a material is to grow the layers by chemical vapor deposition (CVD) onto a single crystal substrate of a different, but readily available material. The process is called heteroepitaxy. Heteroepitaxy takes advantage of the fact that certain single crystal wafers, for example, silicon, are commercially available in large diameters. However, this technique has one major problem that is related to the high temperature (500-1200.degree. C.) required for the CVD process. When the thin film-substrate is cooled down to room temperature after the growth is complete, the difference in the thermal expansion coefficients of the two different materials causes the film-substrate to bow and crack. U.S. Pat. No. 4,368,098, disclosed the deposition of material by the CVD process and is incorporated by reference.
One method of trying to prevent this bowing and cracking has been to grow a buffer layer between the film and the substrate. The paper by R. M. Lum, et al., Appl. Phys. Lett., 51, 36(1987), describes a method for growing gallium arsenide on silicon. It relies on growing a thin semi-amorphous gallium arsenide layer at low temperatures (425.degree. C.) followed by a thicker gallium arsenide layer grown at standard CVD temperatures (about 700.degree. C.). This method is shown to improve the crystalline quality. However, this technique is not totally successful in removing all of the stress induced by the thermal expansion differences.
A second method described by S. Sakai, Appl. Phys. Lett., 51, 1069(1987) involves pre-stressing the substrate in the opposite direction of the thermal expansion difference induced stress. This is accomplished by placing a substrate on a graphite holder with a screw-like push rod pushing against the back of the substrate (See FIG. 2 of the above) until the substrate is bowed. The holder and substrate are then placed in the CVD hot zone, heated up to growth temperature and the film is then grown on the substrate. The holder and substrate are then cooled to room temperature and the substrate is removed. The technique has two main drawbacks. First, it would be difficult to design a reactor injection system that would grow uniformly thick films across the whole wafer. Second, the technique will only work with substrates that are not brittle and break when stressed